1. Field of the Invention
This invention relates to display panels and, more particularly, to display panels comprising a plurality of liquid crystal elements.
2. Description of the Prior Art
In receiving apparatus, it is advantageous to easily identify the carrier frequency of a transmitter that has just been dialed. A pointer is usually used for this purpose. The pointer is fastened to a string pulley arranged to activate a frequency selecting rotary condensor for tuning the receiver to the transmitter carrier frequency. The position of the pointer relative to a scale on the receiver indicates the frequency to which the receiver is tuned.
In more modern receivers, the scale and pointer has been replaced by a display panel comprising Light Emitting Diodes (LED) arranged to indicate the frequency at which the receiver is tuned. As disclosed in a German publication, radio mentor, 10, 1971, page 605, the tuned frequency of the receiver is determined by operating one LED in a display panel having 16 LED's. The position of the operaating LED relative to the other diodes in the display panel determines the tuned frequency of the receiver.
In another prior art receiver, an electronic front plate has been proposed in which LED's likewise indicate the tuned frequency of the receiver. As disclosed in the German publication Elector, Apr. 19, 1973, page 21, preselected LED's are switched on at the same time that a direct current (D.C.) tuning voltage is coupled to a variable capacitance diode, such as a varactor diode. A disadvantage with the LED indicators described above is that they are point-shaped and therefore not easily discernible.
It would also be conceiveable to indicate the operating frequency of the receiver by means of analog controlled liquid crystal elements. A display panel comprising liquid crystal elements is described in Applied Optics,June 1970, Vol. 9, No. 6, page 1323 in an article entitled "Electronically Scanned Analog Liquid Crystal Displays." It is possible that a display panel comprising liquid crystal elements may be controlled by the D.C. tuning voltage applied to a frequency tuning varactor diode. However, is is desirable to operate liquid crystal elements in response to an alternating current(A.C.)voltage in order to provide a relatively long operating lifetime for the liquid crystal elements. Furthermore, an antilogarithmic relationship exists between the turning frequency and the D.C. tuning voltage applied to the varactor diode, so that a non-linear course would result on the display panel. To this must be added that a sharp, definite transition between controlled and uncontrolled regions in a liquid crystal display panel is difficult to attain when the liquid crystal elements operate in response to an analog signal. Thus, in an analog liquid crystal display for receivers, there exists a transition region between light and dark which does not permit exact reading of receiver operating frequency.